1. Field of the Invention
The present invention relates to a display device and, more particularly, to a display device provided with a display panel comprising thin-film transistors (TFT) as switching elements for display pixel selection.
2. Description of the Related Art
Liquid-crystal display devices and organic EL display devices, for example, are known as display devices that comprise display panels which comprise thin-film transistors (TFT) as switching elements for display pixel selection. When the display selection of each pixel is performed with TFT's in such display devices, the voltage polarity of the opposing electrode must be inverted line by line and in each frame in order to decrease the residual image phenomenon, for example. Also, in inverted driving, aside from the effect of decreasing the residual image phenomenon, with frame-inversion driving and horizontal-line inversion driving, and so forth, it is possible to achieve the result of being able to halve the voltage amplitude applied to the signal lines by inverting the polarity of the opposing electrode voltage that is applied to the opposing electrode in sync with the inversion timing.
FIG. 15 shows an example where the polarity of the voltage applied to the opposing electrode is inverted in a display device. The display device selects and drives each pixel that the display panel comprises by sequentially applying a gate signal to the pixels (FIG. 15B). Normally, when the polarity of the opposing electrode is inverted in every frame, the voltage waveform of the opposing electrode is a rectangular waveform that is in sync with a fixed frame frequency (FIG. 15A, for example). In this case, by inverting the polarity of the voltage that is applied to the opposing electrode in every frame, it is possible to reduce the drive voltage and simplify the drive circuit in addition to reducing the residual image phenomenon known as “burn-in”.
The display devices driven by TFT's may be driven in a cycle period which includes two periods, one of which is a scan period in which display is performed by sequentially scanning selecting, and driving the TFT of each pixel according a gate signal, and another of which is a non-scan period in which the TFT's of each pixel are not driven.
FIG. 16 serves to illustrate serves to illustrate the inversion of the voltage waveform of the opposing electrode in cyclical driving that includes a scan period and a non-scan period. In FIG. 16, a single subframe of a field sequential color system (described subsequently) is constituted by a scan period and a non-scan period. Here, the terms ‘frame’, ‘subframe’, and ‘field’ are used with substantially the same meaning. FIG. 16A shows the opposing electrode potential, and FIG. 16B shows the gate signal.
Here, the voltage polarity of the opposing electrode during the scan period is inverted in every subframe.
Japanese Patent Application Laid-open No. 2003-330425 discloses a display device in which the polarity of the voltage of the opposing electrode is inverted in each field in the non-scan period.
In conventional display devices, there is a problem that, when the polarity of the voltage waveform of the opposing electrode is inverted, a leak current is generated and the electrical charge in the pixel will flow out to the source line.
In order to suppress the generation of the leak current, in, for example, the case of N-type TFT elements, it is necessary to raise the absolute value of the gate-drain voltage of the data retention interval (the gate OFF period) above a fixed value by reducing the amplitude of the voltage of either the opposing electrode or the source line. However, when the amplitude of the voltage of either the opposing electrode or the source line is reduced in this manner, the problem that sufficient contrast cannot be obtained arises.
A field sequential color (hereafter abbreviated as ‘FSC’) system, according to which color display is performed as a result of a plurality of different wavelengths of light being sequentially emitted in a predetermined cycle and according to which the liquid crystals are driven in sync with the light source emission timing, is known as a color liquid crystal display system. However, such an FSC system requires a rapid response for the liquid crystals and it is necessary to apply a high voltage for this purpose. Therefore, the problem that an adequate contrast cannot be obtained in cases where the amplitude of the voltage of either the opposing terminal or the source line is reduced in order to suppress the generation of a leak current as mentioned earlier is then more obvious.